湘赣地区中生代镁铁质岩浆作用与岩石圈伸展

MESOZOIC MAFIC MAGMATISM IN HUNAN-JIANGXI PROVINCES AND THE LITHOSPHERIC EXTENSION

综合分析了华南内部中生代 (178～ 80Ma)镁铁质岩石的年代学和元素同位素地球化学特征。研究表明区内主要发育 4期镁铁质岩浆活动 :2 2 0Ma± ,175Ma± ,12 0～ 15 0Ma ,80～ 90Ma ,其中 2 2 0Ma仅在道县发育辉长岩包体。地球化学上区内 175Ma左右的宁远太阳山和赣中项家碱性玄武岩和 80～ 90Ma的镁铁质岩石主要表现为Hawaii OIB玄武岩的元素同位素地球化学特征 ,而175Ma的其他镁铁质岩石表现为岩石圈地幔属性。 12 0～ 15 0Ma的镁铁质岩石则介于岩石圈地幔端员与软流圈地幔端员之间。时、空上表现为以郴州—临武断裂为界 ,西侧自 170Ma左右的EMI型为主向 80～ 90Ma的OIB型为主迁移演化 ,而东侧则自老而新自EMII型 (175Ma)为主演变为以OIB型 (80～ 90Ma)为主。上述资料暗示区内至少存在 4期强烈的岩石圈减薄作用 ,软流圈物质上涌和岩石圈伸展减薄是华南中生代岩浆作用形成的主要机制。但 2 2 0Ma左右的伸展减薄范围局限 ,而较大规模的岩石圈伸展减薄和软流圈上涌始于 178Ma ,其形成可能与华南印支造山作用的后造山 (或后碰撞 )拉张裂解地球动力学背景有关。同时也暗示郴州—临武断裂可能是界定中生代EMI型扬子岩石圈和EMII型华夏岩石圈地幔的重要边界。

The geochronological, elemental and isotopic characteristics of Mesozoic mafic rocks in South China interior are systemically analyzed. Four episodes of Mesozoic mafic magmatism in South China were identified: ca. 220 Ma; ca.175 Ma; 120～150 Ma; 80～90 Ma, respectively. The earliest episode magmatism was only developed as gabbro xenolithes at Daoxian county, Hunan Province. The alkaline basalts with age of ca.175 Ma from Taiyangshan, Ningyuan and Xiangjia, Ji'an, as well as the mafic rocks with age of 80～90 Ma in South China, have Hawaii-OIB elemental and isotopic characteristics whereas other mafic rocks with age of 175 Ma exhibit those of the enriched lithospheric mantle. The mafic rocks with age of 125～150 Ma have variable elemental and isotopic composition between OIB- and EM-type mantles. The nature of source is, both in time and space, gradually transferred from EMI-dominated (ca.175 Ma) to OIB-dominated (ca. 80～90 Ma) at the west of the Chenzhou-Linwu fault whereas from EMII-dominated to OIB-dominated at the east. These data suggest that at least four stages of strong lithospheric extensions were developed in the South China. The asthenosphere uplifting in response to intra-continental lithospheric extension is the important mechanism of Mesozoic magma generation in the South China interior. However, the lithospheric extension with age of ca. 220 Ma might be locally developed and large-scale lithospheric extension and thinning was probably begun at ca. 178 Ma. Additionally, the spatial variations of EMI- and EMII-like source signatures for mafic rocks with age of >120 Ma around the Chenzhou-Linwu fault seems to suggest that the fault represents the Mesozoic lithospheric boundary between the Yangtze and Cathaysia blocks.